Liquid crystal display devices have been widely used as display units for electrical devices including computers, TV sets and cellular phones, but are increasingly required to have further improved display characteristics as well as lower power consumption. A possible approach to meet these requirements is twofold: adequately diffusing light from the light source, and improving the use efficiency of light from the light source. Specifically, when the light emitted from the light source is adequately diffused, the liquid crystal display device is able to offer a wider viewing angle and/or increased in-plane luminance uniformity. High-light use efficiency can realize not only increased luminance of the entire liquid crystal display device for brighter display, but also lower power consumption.
Patent Document 1 discloses a reflective polarizer which allows polarized light beam (a) of specific linear polarization to pass though but reflects polarized light beam (b) with polarization perpendicular to that of light beam (a). Patent Document 1 also discloses a liquid crystal display device which includes this reflective polarizer. The liquid crystal display device includes, in the order from the display-screen surface, a liquid crystal cell, a reflective polarizer, a backlight, and a diffusive reflector.
Among light beams with different polarizations from the backlight, polarized light beam (a) passes through the reflective polarizer as a display light beam, whereas polarized light beam (b) is reflected back by the reflective polarizer as a reflection light beam. Polarized light beam (b) reflected by the reflective polarizer is again reflected by the diffusive reflector while being randomly polarized, whereby the light beam is converted to light containing both polarized light beam (a) and polarized light beam (b). Among light beams of the randomly-polarized light, polarized light beam (a) passes through the reflective polarizer (a) as a display light beam, whereas polarized light beam (b) is again reflected as a reflected light beam. Patent Document 1 discloses that use efficiency of light from the backlight can be enhanced with this configuration. The disclosed reflective polarizer is a multi-layered film consisting of films A made of polyethylene naphthalate and films B made of copolyester prepared using a diacid such as naphthalenedicarboxylic acid or terephthalic acid as an acid component.
As another reflective polarizer, Patent Documents 2 and 9 disclose a sheet made up of a first transparent resin continuous phase dispersed with particles or other forms of a second transparent resin. The sheet similarly allows polarized light beam (a) to pass through and reflects polarized light beam (b) with polarization perpendicular to that of light beam (a). This sheet is prepared by extrusion molding of a mixture of two different resins.
Patent Documents 3 to 5 disclose a light guide film or sheet to which haze anisotropy is imparted. Because only a light beam with specific polarization diffuses through and emits from the film when non-polarized light is incident on the film end (edge), the light guide may increase the use efficiency of the incident light. The disclosed film is prepared by uniaxial stretching of a polyethylene naphthalate film or the like which contains or is free of a filler.
Patent Document 6 discloses a production method of resin articles for container applications, which involves biaxial stretching of non-oriented crystallized resin (e.g., polyethylene terephthalate resin) sheets.
It is known that luminance at 0° viewing angle (normal-direction luminance) is one of the important characteristics for liquid crystal display devices. Patent Documents 7 and 8 disclose, as a means of improving normal-direction luminance, employing an optical film (e.g., reflective polarizer) having prisms on its surface for adjustment of the light emission angle with respect to the film surface.    [Patent Document 1] Japanese Patent Application Laid-Open (Translation of PCT Application) No. 09-506985    [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-075643    [Patent Document 3] Japanese Patent Application Laid-Open No. 11-281975    [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-264539    [Patent Document 5] Japanese Patent Application Laid-Open No. 2001-49008    [Patent Document 6] Japanese Patent Application Laid-Open (Translation of PCT Application) No. 2005-531445    [Patent Document 7] Japanese Patent Application Laid-Open No. 2007-272052    [Patent Document 8] Japanese Patent Application Laid-Open No. 2007-206569    [Patent Document 9] Japanese Patent Application Laid-Open (Translation of PCT Application) No. 2000-506989